Common rail fuel injection systems come in many forms. For instance, a common rail fuel injection system might maintain fuel at injection pressure levels in the common rail, and then inject at that pressure by respective fuel injectors connected to the common rail. In another example, a separate actuation fluid, such as lubricating oil, is maintained in a common rail at a medium pressure level. This actuating fluid is then supplied to individual injectors which utilize the actuation fluid to hydraulically pressurize fuel within the individual injectors to injection pressure levels. In still another example, fuel is maintained in a common rail at a medium pressure level. The individual fuel injectors connected to such a rail have the ability to inject directly at the medium pressure level, or utilize the medium pressure fuel to hydraulically intensify the pressure of the fuel to be injected from the fuel injector. In all of these cases, the fuel injection rate is strongly a function of the rail pressure. Thus, as one would expect, the determination of injection control signals are currently based at least in part upon an estimated rail pressure. The accuracy of any given fuel injection event is strongly related to the accuracy of a rail pressure estimate used in determining the injection control signals that will be used in an attempt to deliver those desired injection characteristics.
Engineers have observed that rail pressure can vary substantially between injection sequences but also within an injection sequence itself. In many cases, these fluctuations in rail pressure can exceed 15% of the average rail pressure, especially, and possibly to a larger extent, during cold starting. These fluctuations in rail pressure can be attributable to a number of phenomena. For instance, localized rail pressure fluctuations can be attributable to pressure waves bouncing around in the common rail due to such events as the opening and closing of various valves. More significantly, however, is the fact that in most cases the common rail is steadily supplied with fluid from a high pressure pump, but fluid is consumed from the rail by the injectors in brief gulps. Thus, one could expect rail pressure to drop with each injection event, and then recover between events. Much more accurate delivery timings and quantities can be achieved if the rail pressure is known at the start of each injection event. Unfortunately, it is currently difficult to instantaneously obtain an accurate rail pressure measurement, and in the same instant generate control signals based upon that rail pressure measurement, and again in that same instant carryout the determined control signal. Thus, one problem associated with improving delivery and timing accuracy of fuel injection events is the problem of accurately determining what the rail pressure will be at the beginning of each one of those events.
In most systems, a single CPU processor is used by the electronic control module to control the engine, which includes the fuel injection system. Since the processor can only do one thing at a time and because the processor is generally occupied with processing data in regular periodic intervals and controlling injector drivers during an injection event, only a limited amount of time is available to determine desired injection characteristics, determine control signals and set up for a subsequent injection event, especially at high speeds and loads. In fact, under some higher speed and load conditions, the time available may be so short as to require the electronic control module to calculate control signals for more than one injection event for two different cylinders. In all of these cases, rail pressure is sensed sometime before the determination of control signals for some subsequent injection event. For instance, in some cases rail pressure is sensed at regular intervals, such as every so many milliseconds. Unfortunately, such a strategy can result in an inaccurate rail pressure being used to determine control signals since the rail pressure measurement might be taken a relatively long period of time before the subsequent injection event. In other cases, a rail pressure measurement is taken at the start of current for an injection event, and then that rail pressure measurement is used as an estimate for determining control signals for one or more later injection events. While such a strategy avoids the possibility of a processor interrupt at an inopportune time, the rail pressure data can be relatively stale since the rail pressure measurement used in calculating control signals is based upon a rail pressure that occurred at the beginning of a preceeding injection event.
The present invention is directed to these and other problems associated with determining rail pressure in a common rail fuel injection system.